Parrotfish maintain the balance between corals and seaweeds. (Image: Science.)

Marine reserves give a boost to coral reefs as well as fish stocks, new research shows.

Scientists had been concerned that large fish returning to protected areas of the Caribbean could disturb the delicate balance of reefs.

They feared that larger predators would eat the smaller fish which graze on coral and keep down harmful algae.

But a study published in the journal Science found that coral in a marine reserve in the Bahamas is flourishing.

Rather than eating all the parrotfishes - the main creatures that clean up the reef - the returning predators, such as the Nassau grouper, only eat the smaller species.

Parrotfish bigger than about 25 cm (10 inches) long are able to escape the predators' jaws, and do a more efficient job at removing algae from coral, stopping it from being smothered.

This is critical for Caribbean reefs, which were hit by the mass death of sea urchins, the main creatures to feed on the algae, in 1983, due to disease.

No-fishing zone

Parrotfish have become the dominant grazers on most Caribbean reefs but numbers have dwindled in areas where fishing is allowed.

Large species of parrotfish flourish in protected areas. (Image: Science.)

This had led to concerns about their fate in marine reserves, which focus on restoring habitat for large predators at the top of the food chain.

"The answer quite clearly is that this is not going to be a problem," said Peter Mumby of Exeter University in the south of England, who led the study.

He said marine reserves have a very beneficial effect for coral and will help reefs withstand the impact of climate change, coral bleaching and other threats.

"This is the first time that anyone has shown that allowing a reserve to be successful in allowing fish to recover can reduce the amount of seaweed on the reef, which (in turn) increases the ability of coral to recover from things like hurricanes, tsunami and coral bleaching," Dr Mumby told the BBC News website.

The study looked at the Exuma Cays Land and Sea Park, which lies near the centre of the Bahamas archipelago. It was established in 1959, and has been a no-fishing zone for 20 years.